1. Field of the Invention
The present invention relates to an apparatus and a method for localizing sound image.
2. Description of the Related Art
Conventionally, a home television (TV) set capable of performing a stereophonic audio reproduction includes a pair of speakers (i.e., a left speaker and a right speaker). However, since such a TV set has a limited width for installing the speakers therein, it is not possible to enjoy stereophonic audio reproduction at satisfactory level. Furthermore, if such a TV set employs a xe2x80x9csurround systemxe2x80x9d, it is often difficult to provide surround speakers.
In such a case, audio signals are subjected to a localization treatment of sound image (e.g., by using a head-related transfer function (HRTF)) and the treated signals are supplied to the speakers, so as to localize sound image (i.e., virtual speakers) at positions where speakers are not actually arranged. The virtual speakers make a listener to feel that the distance between the actually arranged speakers is widen, or to feel that the listener hears reproduced sound from sideward or rearward of the listener although only two frontal speakers are actually arranged in front of the listener.
Generally, in the case of moving sound image, it is relatively easy to localize the sound image at a predetermined position although it depends on a listener. In contrast, in the case of staying sound image, it is difficult to localize the sound image at a predetermined position.
In order to overcome the above-mentioned problem, a technique making a listener to recognize sound image at a predetermined position has been proposed. When the predetermined position is located away at an angle xcex8 in a circumferential direction from the front of the listener, the technique includes producing (i) a first processed signal for localizing sound image at a first localization position located away at an angle xcex81 in a circumferential direction from the front of the listener wherein xcex81 less than xcex8, and (ii) a second processed signal for localizing sound image at a second localization position located away at an angle xcex82 in a circumferential direction from the front of the listener wherein xcex82 greater than xcex8; and alternately supplying the first and the second processed signals to the speakers, so as to alternately localize sound image at the first and the second localization position for making the listener to recognize sound image at the predetermined position.
However, such a technique provides the listener with a quite unnatural feeling of hearing due to the regularity of the alternate sound image localization around the predetermined position.
Next, the case of moving sound image will be described.
An apparatus, wherein a pair of speakers are arranged at positions left and right front sides of a listener and wherein a single audio signal is divided into two branched signals to be supplied to the respective speakers, is capable of moving sound image in a left or right direction between the speakers. The sound image movement is accomplished by, for example, continuously increasing an amplitude (a level) of one of the branched signals as well as continuously decreasing an amplitude of another branched signal.
However, in the case of simply increasing and decreasing the amplitude of the branched signals, a listener often feels that the sound image is moving in an area rearwards to the speakers when the sound image is located at the middle between the speakers. In order to make the listener to feel that the sound image is moving in a left or right direction between the speakers, the following procedure is conventionally employed.
(i) When sound image is located at the middle between the left and right speakers, the procedure includes increasing an amplitude of the branched signals in a small amount, respectively. (ii) When sound image is moving from left or right side to the middle between the speakers, the procedure includes shifting a frequency component to high frequency side in advance, and then returning the shifted component to an original one as sound image is moving to the middle between the speakers. In contrast, when sound image is moving from the middle between the speakers to left or right side, the procedure includes shifting a frequency component to low frequency side in advance, and then returning the shifted component to an original one as sound image is moving to left or right side. In other words, the procedure includes incorporating the Doppler effect. Alternatively, (iii) when sound image is moving from left or right side to the middle between the speakers, the procedure includes virtually increasing a high frequency component of the branched signals and decreasing a low frequency component thereof. In contrast, when sound image is moving from the middle between the speakers to left or right side, the procedure includes virtually increasing a low frequency component of the branched signals and decreasing a high frequency component thereof.
As described above, it is relatively easy to make a listener to feel that sound image is moving in a left or right direction. However, it is difficult to make a listener to feel that sound image is moving forward and backward with respect to the listener by using only two speakers (i.e., the left and right speakers).
For example, when sound image is approaching a listener, it is possible to make the listener to feel that the sound image is approaching the listener to some extent, by gradually increasing an amplitude of the branched signals. Especially, when a picture image is accompanied with the sound image, such a feeling may be emphasized. However, it is not possible to make a listener to feel that sound image is approaching the listener sufficiently or moving rearwards with respect to the listener.
In order to overcome the above-mentioned problem, the below-indicated technique has been proposed. As shown in FIG. 26, when branched signals supplied to a left speaker 211 and a right speaker 212 have the same phase (i.e., the correlation is 1), a listener 214 feels that sound image 213 is located at the position 220 rearwards of the middle between the speakers 211 and 212; when the phase difference between the branched signals is 90 degrees (i.e., the correlation is zero), a listener 214 feels that sound image 213 is widen in an area 221 between the speakers 211 and 212; when the phase difference between the branched signals is 180 degrees (i.e., the correlation is xe2x88x921), a listener 214 feels that sound image 213 is located at an area 222 rearwards to the listener 214. The technique includes moving sound image 213 forward and backward with respect to the listener by varying the phase difference between the branched signals (i.e., by using a relationship shown in FIG. 26).
However, even when the above-mentioned technique is utilized, it is not possible to make a listener 214 to clearly feel that sound image 213 is moving forward and backward with respect to the listener.
As described above, an apparatus and a method for localizing sound image which provide a natural feeling of hearing is eagerly demanded.
The present invention includes the steps of providing a left speaker and a right speaker in front of a listener; subjecting an audio signal to a sound image localization treatment, so as to produce a processed signal; and supplying the processed signal to the left and the right speakers, so as to localize sound image at a predetermined position. Wherein the method further includes: producing a first processed signal which localizes sound image at a first localization position and a second processed signal which localizes sound image at a second localization position; multiplying one of the first and the second processed signals by a coefficient k which varies in the range of 0 to 1; multiplying the other signal by a coefficient 1xe2x88x92k; and adding the processed signal multiplied by the coefficient k and the processed signal multiplied by the coefficient 1xe2x88x92k. When the predetermined position is located away at an angle xcex8 in a circumferential direction from the front of the listener, the first localization position is in the vicinity of the predetermined position and located away at an angle xcex81 in a circumferential direction from the front of the listener wherein xcex81 less than xcex8, and the second localization position is in the vicinity of the predetermined position and located away at an angle xcex82 in a circumferential direction from the front of the listener wherein xcex82 greater than xcex8.
In one embodiment of the invention, a spectrum of the coefficient k has 1/f characteristics.
In another embodiment of the invention, a production of the coefficient k includes outputting a random signal having rectangular pulse shape, height of 1, and random pulse width and pitch, and integrating the random signal in an integration circuit.
In still another embodiment of the invention, a production of the coefficient k includes squaring the audio signal by a squaring circuit, and processing the squared signal through a low pass filter.
In still another embodiment of the invention, the audio signal is a 2-channel stereophonic signal, and a signal for producing the coefficient is selected from a signal of one of the channels, an added signal of the both channel, or a differential signal of the both channel.
According to another aspect of the present invention, an apparatus for localizing sound image is provided. The apparatus includes: a left and a right speakers to be provided in front of a listener; a means for subjecting an audio signal to a sound image localization treatment so as to produce a processed signal; and a means for supplying the processed signal to the left and the right speakers so as to localize sound image at a predetermined position. Wherein the apparatus further includes: a means for producing a first processed signal which localizes sound image at a first localization position; a means for producing a second processed signal which localizes sound image at a second localization position; a means for producing a coefficient k which varies in the range of 0 to 1; a means for multiplying one of the first and the second processed signals by the coefficient k; a means for multiplying the other signal by a coefficient 1xe2x88x92k; and a means for adding the processed signal multiplied by the coefficient k and the processed signal multiplied by the coefficient 1xe2x88x92k and supplying the added signal to the left and the right speakers. When the predetermined position is located away at an angle xcex8 in a circumferential direction from the front of the listener, the first localization position is in the vicinity of the predetermined position and located away at an angle xcex81 in a circumferential direction from the front of the listener wherein xcex81 less than xcex8, and the second localization position is in the vicinity of the predetermined position and located away at an angle xcex82 in a circumferential direction from the front of the listener wherein xcex82 greater than xcex8.
According to still another aspect of the present invention, a method for moving sound image is provided. The method includes the steps of: producing a single audio signal; dividing the single audio signal into two branched signals; shifting a frequency component of the audio signal or the branched signals; amplifying an amplitude of the audio signal or the branched signal; varying a phase difference between the branched signals; and supplying the branched signals to a left and a right speakers. The combination of the shift of the frequency component, the variation of the amplitude and the variation of the phase difference makes a listener to feel that sound image is moving forward and backward with respect to the listener.
In one embodiment of the invention, the combination comprises the steps of: increasing the amplitude of the branched signals; increasing the phase difference between the branched signals from zero degree to 180 degrees; decreasing the amplitude of the branched signals to approximately zero while keeping the phase difference approximately at 180 degrees; and shifting the frequency component of the branched signals to low frequency side.
In another embodiment of the invention, the combination comprises the steps of: keeping the phase difference between the branched signals approximately at 180 degrees while keeping the amplitude of the branched signals identical to each other; decreasing the amplitude and the phase difference to approximately zero; and shifting the frequency component of the branched signals to low frequency side.
According to still another aspect of the invention, an apparatus for moving sound image is provided. The apparatus includes: a source which produces a single audio signal; a means for dividing the single audio signal into two branched signal; a means for shifting a frequency component of the audio signal or the branched signals; a means for amplifying an amplitude of the audio signal or the branched signal; a means for varying a phase difference between the branched signals; and a left and a right speakers to which the branched signals are respectively supplied. The combination of the shifting means, the amplifying means and the phase difference varying means makes a listener to feel that sound image is moving forward and backward with respect to the listener.
Thus, the invention described herein makes the possible the advantages of: (1) providing an apparatus for localizing sound image which provides a natural feeling of hearing; and (2) a method for localizing sound image which provides a natural feeling of hearing.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.